1. Field of the Invention
The present invention relates to a method of straightening rolled sectional steel.
2. Description of the Related Art
Cooling of rolled sectional steel, for example, I-sections and U-sections or angles, usually takes place on a cooling bed. Because of non-uniform cooling, the sections become distorted. This distortion has a negative effect on the straightness and internal stress state of the sections. Taken together, these two quality criteria can be compared to the quality criterion flatness in strip rolling. A reduced straightness (section curvature, twist and bending curvature) frequently occurs when high internal stresses occur. Curved sections must be further processed. Internal stresses reduce the load bearing capacity of the sections.
In accordance with the prior art, when curvatures occur they are returned at low section temperatures by means of one or more straightening processes to a tolerable extent. Used for this purpose are roller straightening machines and straightening presses.
Roller straightening machines which continuously straighten the sections, initially produce another curvature of the section to a defined dimension. As this occurs, the existing internal stresses are eliminated by new defined internal stresses. However, this is inherently not possible over the entire cross-section of the section. In the area of the neutral fiber, a material area remains which is not influenced over the entire straightening process. After the first bending process has occurred, the product is subjected to a defined alternating bending with several changes of the curvature. This changes the internal stresses in such a way that the section is straight after the straightening process. Inherently, residual internal stresses remain. The internal stresses remaining in the sectional steel are a disadvantage because of the already mentioned problems with respect to the load bearing capacity of the sections. Sections with substantial curvatures additionally pose problems during the straightening process, for example, the threading-in into the machine.
In the discontinuously operating straightening press, individual portions of the sectional steel which are impermissibly strongly curved are one after the other compensated by a bending process which is as much as possible the opposite of the curvature. When using the straightening press, it is not possible to influence the internal stress state. The discontinuous and unknown internal stress state after the straightening process has a disadvantageous effect on the load bearing capacity of the section. This process harmfully influences the material flux during the manufacture of sectional steel and requires a lot of time.